A ferritic stainless steel has excellent oxidation resistance and cold workability, and, on the other hand, its high temperature strength is inferior to that of an austenitic stainless steel. For this reason, the ferric stainless steel is not so much employed as a heat-resistant strength member. As the most common uses, the ferritic stainless steel is used in a muffler, a pipe and the like involving thermal fatigue, utilizing its low coefficient of thermal expansion. Furthermore, a ferritic stainless steel containing Mo and Nb is liable to form Laves phase after melting and casting or when exposed to high temperature. Coarse Laves phase causes deterioration of toughness and workability. To expand uses of the ferritic stainless steel, those problems must be overcome. In view of the above, various proposals have been conventionally made to overcome those problems.
For example, Patent Document 1 discloses a method in which a ferritic Cr-containing steel material containing a predetermined amount of W is subjected to hot rolling, the hot rolled plate is subjected to annealing and cold rolling, followed by finishing annealing at a temperature of 1,020 to 1,200° C.
This Patent Document describes that (A) the amount of W precipitated can be decreased to 0.1% or less by the method and thereby, (B) a coefficient of thermal expansion of an alloy can be remarkably decreased.
Patent Document 2 discloses a method for manufacturing an Nb-containing ferritic stainless steel hot rolled and annealed coil including (a) hot rolling a slab containing an Nb-containing ferritic stainless steel at a finishing rolling temperature of 890° C. or higher, (b) cooling the resulting hot rolled sheet strip with water and taking up the sheet strip at a winding temperature of 400° C. or lower to form a coil, and (c) dipping the coil after taking up at low temperature in water.
This Patent Document describes that (A) brittleness due to the formation of Laves phase and 475° C. brittleness occur by heat recuperation after taking up into the coil even though the sheet strip is merely cooled with water and (B) when the sheet strip is taken up at a temperature of 400° C. or lower and the resulting coil after taking up is dipped in water, heat recuperation and brittleness due to the heat recuperation can be suppressed.
Patent Document 3 discloses a method for manufacturing a heat-resistant ferritic stainless steel sheet, including (a) hot rolling a slab containing a Cu-containing heat-resistant ferritic stainless steel to obtain a hot rolled coil, (b) cold rolling the hot rolled coil, and (c) annealing the cold rolled sheet at a temperature of 980° C. to 1,070° C.
This Patent Document describes that (A) when Cu is added, high temperature strength is improved, but oxidation resistance greatly varies due to slight difference of components, and (B) when components are optimized, the formation of γ phase in a surface layer part during maintaining at high temperature is suppressed, and deterioration of oxidation resistance can be suppressed.
Patent Document 4 discloses a method for manufacturing a ferritic stainless steel including (a) hot rolling a ferritic stainless steel containing 0.3 mass % or more of Nb, (b) cold rolling the hot rolled sheet, and (c) finally annealing the cold rolled sheet at a temperature of 1,000° C. to 1,100° C.
This Patent Document describes that (A) when Ni brazing is applied to a ferritic stainless steel, the material must be exposed to high temperature of 1,100° C. or higher, but at such high temperature, the ferritic stainless steel causes coarsening of grains and toughness is liable to be deteriorated, and (B) when 0.3 mass % or more of Nb is added, coarsening of grains at Ni brazing temperature can be suppressed.
Patent Document 5 discloses a heat-resistant ferritic stainless steel having optimized contents of Al, Ti and Si.
This Patent Document describes that (A) a ferritic stainless steel is easy to cause internal grain boundary oxidation when used at high temperature, and (B) when solute amounts of Al and Ti contained in a ferritic stainless steel are limited and the amount of Si added is increased, internal grain boundary oxidation can be suppressed up to a temperature region of 900° C.
Improvement in heat resistance of a ferritic stainless steel is generally achieved by solid-solution hardening by the addition of Mo, but the addition of Mo in large amount is suppressed from the standpoints of avoiding deterioration of workability and cost reduction. On the other hand, W has been known as an element having the same effect as Mo, and a material in which a part of Mo is substituted with W has been proposed (see Patent Document 1). However, a ferritic stainless steel to which only W was added for the purpose of improving heat resistance has been not almost proposed. This is because solid-solution hardening ability of W is small as compared with that of Mo, and large amount of W must be added in order to obtain the same degree of strength. Furthermore, no ferritic stainless steel substantially containing only W as an element for solid-solution hardening and excellent in cold workability and heat resistance has been conventionally proposed.
Patent Document 1: Japanese Patent No. 4604714
Patent Document 2: JP-A 2012-140688
Patent Document 3: JP-A 2009-235555
Patent Document 4: JP-A 2009-174040
Patent Document 5: JP-A H08-170155